On Mon, May 12, 2014 at 06:36:57PM -0500, tthayer@xxxxxxxxxx wrote:
> + ptemp[0] = 0x5A5A5A5A;
> + ptemp[1] = 0xA5A5A5A5;
> + /* Clear the error injection bits */
> + regmap_write(drvdata->mc_vbase, CTLCFG, read_reg);
> + /* Ensure it has been written out */
> + wmb();
> +
> + /*
> + * To trigger the error, we need to read the data back
> + * (the data was written with errors above)
> + * The ACCESS_ONCE macros are used to prevent the
> + * compiler optimizing these reads out.
> + */
> + reg = ACCESS_ONCE(ptemp[0]);
> + read_reg = ACCESS_ONCE(ptemp[1]);
> + /* Force Read */
> + rmb();
Right, I still am a bit unsure about this thing. So sure, we funnel
ptemp through an asm() block which stops the optimizer but we assign the
results to reg and read_reg, i.e. two local variables which aren't used
in this function anymore.
Thus, I don't see what stops the compiler from discarding them along
with the ACCESS_ONCE() reads at some point, when it becomes really
smart. Basically killing
> + reg = ACCESS_ONCE(ptemp[0]);
> + read_reg = ACCESS_ONCE(ptemp[1]);
as they have to effect.
Maybe we can do the reads in asm - this should be pretty safe.
I.e., something like this:
asm volatile("" : "=r" (reg), "=r" (read_reg),
: "0" (ptemp[0]), "1" (ptemp[1]));
and it does it on x86 (did a small test program) by shuffling the values
through registers so we definitely have the reads.
.loc 1 28 0
movl -48(%rbp), %edx # ptemp, D.2240
movl -44(%rbp), %eax # ptemp, D.2240
.loc 1 27 0
movl %edx, -28(%rbp) # reg, reg
movl %eax, -32(%rbp) # read_reg, read_reg
Btw, if you only want to do the reads, you can simply tell gcc to put them in
registers
asm volatile("" :: "r" (ptemp[0]), "r" (ptemp[1]));
and be done with it but I don't know how smart it is with register
allocation so as to recognize that they're already in registers (when
they already are in some registers) and discard the reads.
Fun stuff! :-)
--
Regards/Gruss,
Boris.
Sent from a fat crate under my desk. Formatting is fine.
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